feat(office_worker): add new exercise for rust exams

subjects/office_worker/README.md

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+## office_worker
+
+### Instructions
+
+Create a structure `OfficeWorker` having the following public fields:
+- `name` as `String`.
+- `age` as `u32`.
+- `role` as `WorkerRole`.
+
+Create an enum `WorkerRole` which can be `Admin`, `User` or `Guest`.
+
+Implement for both the trait `From<&str>`. For `OfficeWorker` the string will have the format `"name,age,role"`, for `WorkerRole` the format of the string will be the `"role"` name in lower case.
+
+### Expected Functions and Data Structures
+
+```rust
+use crate::OfficeWorker::*;
+
+#[derive(Debug, PartialEq, Eq)]
+pub struct OfficeWorker {
+}
+
+#[derive(Debug, PartialEq, Eq)]
+pub enum WorkerRole {
+}
+
+impl From<&str> for OfficeWorker {
+}
+
+impl From<&str> for WorkerRole {
+}
+```
+
+### Usage
+
+Here is a program to test your function.
+
+```rust
+use office_worker::*;
+
+fn main() {
+    println!("New worker: {:?}",
+        OfficeWorker::from("Manuel,23,admin"));
+    println!("New worker: {:?}",
+        OfficeWorker::from("Jean Jacques,44,guest"));
+}
+```
+
+And its output:
+
+```console
+$ cargo run
+New worker: OfficeWorker { name: "Manuel", age: 23, role: Admin }
+New worker: OfficeWorker { name: "Jean Jacques", age: 44, role: Guest }
+$
+```